It is known that even if the use of hot chamber pumps, in which the injection pump is totally or partially immersed in the molten alloy, solves most of the problems of cold chamber pumps, yet it presents the great drawback that when said alloy at melting temperature is corrosive for the ferrous materials, the members of the pumps are rapidly etched by it.
An example of a traditional hot chamber pump dating back to 1940 is disclosed in DE-C-745.583 relating to an improved arrangement for the alignment of the injection piston with the cylinder abutting against the flat top of the gooseneck. This traditional type of pump does not allow high injection pressure and is not suitable for corrosive alloys.
The continuous research of new corrosion-resistant materials, capable of assuring a sufficient life and reliability to the parts exposed to the contact with the corrosive alloys, has led to the development of alloys of various elements such as titanium, boron, silicon, carbon, chromium and aluminum and rarer elements such as yttrium, lanthanum, scandium, cesium, samarium, zirconium, etc. The aim of the research of alloys more and more corrosion-resistant is that of extending the operating life of the pump, mainly as far as the most critical members such as the piston and the cylinder are concerned, which are not only subject to the corrosion by the molten alloy, but they also have to withstand the abrasion caused by the motion of the piston sealably sliding in the cylinder.
In conventional pumps, the play which occurs between piston and cylinder owing both to the thermal expansion and the surface corrosion is extremely damaging for the correct working of the pump. In fact, the introduction of the molten alloy into the cylinder usually takes place through an opening in the side wall of the cylinder which is closed by the piston in its downward stroke with the consequent impossibility of using low rigidity piston rings which would be damaged by the passage on the side opening. FR-A-1.178.540 discloses an example of such a pump, wherein the replacement of the members undergoing corrosion and wear is easy, fast and economical. However this pump is designed for the casting of magnesium alloys, which are not corrosive for the types of metallic materials used nowadays and allow the use of elastic piston rings
In other cases, such as in patents CH625.439, U.S. Pat. Nos. 3,467,171 and 3,469,621, the piston has its lower end cut at 45.degree. or somehow machined to obtain therein a loading mouth so as to allow the inflow of the molten alloy into the cylinder without extracting completely the piston and without forming openings in the side wall of the cylinder. Nonetheless, the piston must sealably slide in the cylinder, and therefore the problem of the coupling tolerances between piston and cylinder remains. Even if metallic piston rings can be applied in this case in order to improve the sealing, said rings wear down rather rapidly thus requiring the replacement thereof after few thousands of cycles. Moreover, their presence implies a limitation of the maximum operating pressure, so as to prevent excessive friction and wear, which in some cases is insufficient to obtain casts of the required compactness.
The maximum pressure may be considerably limited also by sealing problems between the container cylinder wherein the injection piston slides and the seat of the gooseneck siphon wherein said cylinder is housed. This occurs especially if said members are made of different materials, such as in the typical case of a cylinder made of corrosion-resistant ceramic material and a siphon made of coated steel. A further problem stems from the fragility of said ceramic materials which are sensible to bending stresses.
From the above it is apparent that in prior art pumps special surfacings are needed for the critical coupling between piston and cylinder, in which account must be taken of the problems of thermal expansion, friction between the parts, corrosion of the contacting surfaces and possible oxide scales on said surfaces. Similar problems arise in the coupling area between cylinder and siphon, and the whole of these problems implies a shortening of the life of the above-mentioned critical members of the pump with consequent costs, both in terms of pieces replacement and machine stop times for the inspection and/or maintenance thereof.
The applicant has already been granted the U.S. Pat. No. 5,385,456 which discloses a hot chamber pump with a plunger piston. In this way, the cylinder is integral with the siphon, and the sealing is not performed between piston and cylinder but through seals of compressed yielding material located at the mouth of the siphon. Though it substantially solves several of the above-mentioned problems, said pump has limited achievable pressure and injection speed due to the presence of said yielding materials. In fact, it is necessary to limit the pressure in order to prevent an excessive expansion of said materials in the direction transverse to the lateral surface of the piston, in addition to limiting the maximum piston speed in order to prevent an excessive heat production due to the friction.